1. Field of the Invention
The present invention relates to an optical lens, and more particularly, to an optical lens that is capable of minimizing the reduction of the light intensity around a light axis in a process of diffusing the light emitted from a light emitting diode, so that the luminance distribution of an irradiated member can be uniformly obtained and the brightness of the light can be improved.
2. Background of the Related Art
Generally, a surface light source device is used as an irradiation device for a liquid crystal display monitor used in a personal computer or television, and the surface light source device makes use of a plurality of light emitting diodes (hereinafter, referred to as “LED”) as the point sources of light.
The surface light source device includes a plate-like luminous flux control member (optical lens) having the almost same shape as that of a liquid crystal display and the plurality of LEDs arranged to the shape of a matrix on the back surface of the luminous flux control member.
The light emitted from the LEDs is incident on the interior of the luminous flux control member from the back surface of the luminous flux control member and then emitted from an emitting surface opposite to the back surface of the luminous flux control member.
Next, the liquid crystal panel is back-lighted through the emitted light.
Examples of the conventional surface light source devices will be discussed hereinafter.
A first conventional art is disclosed in Japanese Patent Application Laid-open No. 2002-49326. According to the first conventional art, as shown in FIG. 1a, a surface light source device 10 has microlens arrays 12 disposed correspondingly to a plurality of LEDs 11, so that the light emitted from the LEDs 11 is emitted in the direction perpendicular to the plane, that is, upwardly, through the microlens arrays 12.
A second conventional art is disclosed in Japanese Patent Application Laid-open No. 59-226381. According to the second conventional art, as shown in FIG. 1b, a light emitting display device 13 includes an LED 11, a concave lens 14 and a convex lens 15.
The light emitted from the LED 11 is diffused by means of the concave lens 14, collected by the convex lens 15, and then emitted in the almost parallel direction to the light axis of the LED 11.
Herein, the light axis means the advancing direction of light from the center of the luminous flux of the light stereoscopically emitted from the LED 11 as the point source of light.
A third conventional art is disclosed in Japanese Patent Application Laid-open No. 63-6702. According to the third conventional art, as shown in FIG. 1c, a display device 16 includes an LED 11 as the source of light.
The light emitted from the LED 11 is collected and induced forwardly by a light collecting lens 17 and then diffused by a diffusion lens 18.
A fourth conventional art is shown in FIG. 2. As shown, a display device 20 includes a plurality of LED chips 21, a light diffusion member 22, and an irradiated member 23 (for example, a liquid crystal display panel) on which the light transmitted through the light diffusion member 22 is irradiated.
The LED chips 21 are equally spaced apart from each other, and each LED chip 21 is formed by attaching a luminous flux control member 25 having a semi-spherical emitting surface 24 to an LED 26.
The light emitted from each LED chip 21 is transmitted through the light diffusion member 22 and supplied to the irradiated member 23.
However, the above-mentioned conventional arts have the following problems.
Firstly, in case of the surface light source device 10 according to the first conventional art, the microlens arrays 12 are not continuously maintained in shape in the intermediate portion between the neighboring LEDS 11.
Accordingly, the light intensity of the light emitted from the intermediate portion is drastically changed, thereby causing the deviation of the brightness of the light emitted to seriously occur on the boundary portion between the neighboring microlens arrays 12.
Secondly, in case of the second conventional art, when a plurality of concave lenses 14 is continuously connected to each other in the light emitting display device 13, the concave lenses 14 do not form any plane.
Further, when a plurality of convex lenses 15 is continuously connected to each other in the light emitting display device 13, the convex lenses 15 do not form any plane.
Accordingly, for example, it is hard to supply uniform surface irradiation to the member having a large area like a large-sized liquid crystal display panel.
Thirdly, in case of the display device 16 according to the third conventional art, the light emitted from the LED 11 is collected by the light collecting lens 17 and then diffused by the diffusion lens 18.
Accordingly, the difference of the brightness of the light does not appear better than that in the first conventional art.
However, it is difficult to mix the light emitted from the neighboring LEDs 11 with each other, which causes the deviation of the light colors emitted from the LEDs 11 to seriously occur.
Thirdly, in case of the display device 20 according to the fourth conventional art, the luminance deviation of the light emitted from each LED 26 becomes increased in the shape of a waveform as shown in FIG. 3.
Accordingly, the light emitted from each LED 26 becomes dark in a space between the neighboring LEDs 26, which fails to provide uniform surface irradiation.
According to the fourth conventional art, as shown by a curve A in FIG. 3, the light intensity of the light emitted from each LED chip 21 is locally increased around the light axis L of the LED 26.
As a result, it is difficult to mix the light emitted from the neighboring LEDs 26 with each other, which causes the deviation of the light colors emitted from the plurality of LEDs 26 to seriously occur.
So as to solve the above-mentioned problems, accordingly, Korean Patent Registration Nos. 10-0971639 and 10-0977336 disclose a new luminous flux control member (optical lens) which diffuses the light emitted from the point sources of light, mixes the light emitted from the neighboring LEDs with each other, and decreases the deviation of the light colors emitted from the LEDs.
As appreciated therefrom, if the LED is used for wide angle irradiation, the optical element (luminous flux control member) converting the light emitted from the LED into the light diffused to a wide angle is generally disposed in front of the LED.
That is, the light emitted from the LED is emitted through the optical element in accordance with the refraction index of the material of the optical element and Snell's law, and thus the light is converted into the light diffused to a wide angle.
The luminous flux control member converts the light emitted from the LEDs as the point sources of light into the light having the diffusion of about 120° with respect to the vertical lines (light axis) on the center of the LEDs.
That is, a portion where the light intensity of the light emitted from the LED is strong, that is, the light in the range of an angle a in FIG. 4 is diffused to the range of an angle c in FIG. 4.
However, the conventional luminous flux control member (optical lens) has the following problems.
The light emitted from the LEDs as the point sources of light is diffused to a wide angle, but after the light in the range of the angle a is diffused to the wide angle having the range of the angle c, the light intensity may be decreased as shown in FIG. 5.
Accordingly, the intensity of the light is not optimized, which undesirably makes the efficiency deteriorated, and if the optical lens is used for back-lighting, a diffusion plate like a prism sheet should be additionally needed to improve the intensity of the light, which increases manufacturing cost and thus provides economical disadvantages.